Early this year I finally got weary of dragging my collection of mp3s from one iTunes installation to the next. I'd worked out a fairly nice solution where I had the library on a 2TB SSD connected to my laptop's dock, so the music collection didn't take up any space on the laptop. And then a MacOS update— I'm not sure which or what part—changed something so that the Music app, or whatever the successor to iTunes is called, wasn't able to mount a library on an external hard disk.
So I installed Plex on my Raspberry Pi, got a new SSD to be the backup disk for my laptop, and plugged the old SSD with my music library into the Pi and mounted it.
This worked ok for about a week, and then the Plex server started crashing. It turns out that in addition to the media library, Plex keeps its own set of directories where it caches and transcodes things, and it will fill up the spare room on a Pi's SD card pretty fast.
The way to fix this was to make the Plex server keep all of its storage on the portable hard disk. The simplest way to do this seemed to be to symlink where Plex expected to find its directories to somewhere on the SSD: I say "seemed" because to get that to work, I needed to reformat the disk with Linux and add an entry for it in the Pi's fstab. I'd been lazy the first time and used the Pi's ability to mount Mac or Windows drives over USB, but when it does that, they're read-only, and you also can't symlink to them.
But, once that was sorted, I moved the contents of /var/lib/plexmediaserver/Library/Application Support/Plex Media Server to the SDD, put a symlink from the old
location to the new one, and it's been working fine ever since.
I wish I'd written all of that down in February, because I had to spend about half an hour tonight working out what I'd done.